Conventional die casting apparatus are classified into cold chamber and hot chamber. In cold chamber die casting apparatus, molten metal is poured into a sleeve which is secured on a die plate and connected to an inlet opening to the mold cavity. Molten metal is injected by a plunger into the die. The molten metal in the sleeve is easily cooled down when it spreads at the bottom of the sleeve as the plunger moves forward slowly to discharge air or gas. Cooled molten metal in the sleeve forms a chilled fraction and semi-solid or solid particles. The chilled fraction and particles are injected into the molding die causing the physical properties of molded parts to be deteriorated.
Cooled molten metal increases the viscosity of the molten metal and makes it difficult to fill the mold cavity. Further, it causes blemishes on surface of a molded part. This is a serious problem particularly for magnesium alloys for which the latent heat of solidification is small (smaller than aluminum, lead and zinc). Because of the small latent heat of solidification, magnesium solidifies quickly when it comes in contact with materials having a lower temperature.
Hot sleeves have been used, but the heated sleeve is not as hot as liquidus temperature of the metal because the sleeve is connected to a molding die whose temperature has to be below the solidus temperature of the metal. The molding die temperature must be sufficiently below the solidus temperature of the molten metal to produce an adequate solidification rate. That is, a solidification rate which reflects the required time for an operation cycle. Molten metal poured into the sleeve has a substantially higher temperature than the liquidus temperature of the metal to counter the cooling in the sleeve. This is a disadvantage in energy cost for heating.
The cold chamber apparatus forms a thick round plate as a part of the casting, often called a biscuit, in the sleeve between a plunger head and an inlet of a die. After the casting is pulled away from the molding dies when the dies are opened, the biscuit is cut away from the casting and recycled. However, sometimes the biscuit is larger than the product. This is a disadvantageous use of metal which has a substantial recycling cost.
In hot chamber die casting apparatus, an injection mechanism is submerged in molten metal in a furnace. The temperature of the molten metal to be injected is maintained above its liquidus. The injection mechanism has a shot cylinder with a plunger, gooseneck chamber and a nozzle at the end of thereof. The molten metal is injected through a gooseneck-type passage and through a nozzle into the die cavity without forming a biscuit. This is an advantage of hot chamber die casting apparatus.
Another advantage of a hot chamber apparatus over a cold chamber apparatus is the time for an operation cycle. As mentioned above, in cold chamber apparatus, the casting is formed by injecting molten metal into a mold cavity between closed dies and cooling to until the casting is solid. The dies are separated and the molded part is pulled away, lubricant is sprayed onto the opened dies, and the dies are closed again. Then, the dies are ready to start the next operation cycle. The molten metal is poured into the injection sleeve when the molding dies are closed, i.e., when the dies are ready to start the next operation cycle, so that the molten metal does not spill out from the inlet opening of the die because the injection sleeve directly communicates with a die.
On the other hand, hot chamber die casting apparatus fill molten metal in the gooseneck and a shot cylinder system by returning an injection plunger to its fill up position. Molten metal is supplied through an opening or fill port on a shot cylinder. While cooling the injected molten metal in the dies, the nozzle is positioned by inclining the gooseneck chamber. The molten metal in the nozzle gooseneck system tends to flow back into the furnace through the fill port on the shot sleeve, reaching a hydrostatic level when the dies are opened. By simultaneously filling molten metal into the gooseneck and a shot cylinder system and cooling injected metal in the closed dies, time for an operation cycle of the hot chamber apparatus is shortened compared with the cold chamber die casting apparatus.
However, solidification of the molten metal in the nozzle section of the gooseneck and dripping of molten metal from the nozzle and the cast sprue are problems for hot chamber die casting apparatus. It is known that in hot chamber die casting apparatus a vacuum is created in the injection mechanism when the plunger is withdrawn. However, the vacuum is instantaneously destroyed once the plunger passes the opening or fill port on the shot cylinder supplying molten metal from the furnace because the furnace is at atmospheric pressure. Thus, the molten metal is sucked into the shot cylinder, and the gooseneck and the nozzle are completely filled at the time that the casting is solidified and the dies are separated.
There is molten metal in the nozzle for most of the time that the casting is cooling. When the cooling at the tip of the nozzle is properly controlled, it is understood in the industry that the metal in the nozzle tip becomes semi-solid. The formed semi-solid metal works as a plug which prevents molten metal from dripping out of the nozzle when the dies are separated. If the cooling is insufficient, the metal in the tip of the nozzle and the cast sprue is still liquid when the dies are separated and dripping occurs. On the other hand, when too much cooling is applied, the metal in the nozzle tip solidifies and freezes together with the cast sprue. The casting will stick in the stationary die after the dies open.
U.S. Pat. Nos. 3,123,875, 3,172,174, 3,270,378, 3,474,875 and 3,491,827 propose creating a vacuum in the gooseneck by return or reverse stroke of the plunger to draw back molten metal from the nozzle and extreme tip of the sprue. These patents disclose mechanisms attached to the shot cylinder and a plunger system so that the created vacuum is kept intact until after the dies have been separated and the solidified casting has been withdrawn from the sprue opening of the stationary die.
Problems in the hot chamber die casting apparatus are caused because a heavy injection mechanism is submerged in the molten metal in the furnace. The injection mechanism with a gooseneck chamber and a shot cylinder system is difficult to clean up. It is also difficult to replace worn plunger rings and sleeves. A worn plunger ring and sleeve decreases injection pressure due to leakage and makes shot volume inconsistent in filling the mold cavity. The inconsistent shot volume produces inconsistent molded parts.
Die casting apparatus are also classified according to the arrangement of the injection system, that is, horizontal and vertical. In a horizontal die casting apparatus, an injection system is horizontally arranged for horizontally injecting molten metal into molding dies. A vertical die casting apparatus has a vertically arranged injection system for vertical injection of molten metal.
Conventional vertical die casting apparatus typically are vertically arranged cold chamber apparatus that have the same advantages and disadvantages of the cold chamber apparatus described above. However, a feature of the vertical die casting apparatus is that the inlet opening for molten metal can be on top of the vertical injection chamber. This arrangement is not applicable to the horizontally arranged apparatus. In U.S. Pat. Nos. 4,088,178 and 4,287,935, Ube discloses machines in which a vertical casting sleeve is pivotally mounted to a base and slants from perpendicular position to accept molten metal. In place of supplying molten metal to the casting sleeve, Nissan Motors discloses in U.S. Pat. No. 4,347,889 a vertical die casting machine in which a vertical casting sleeve moves downward and a solid metal block is inserted. The inserted metal block is melted in the sleeve by an high frequency induction coil. The problem with these apparatus is the complexity of their structure.